The ruling was hailed as a victory by the pro-life movement, who called the legislation an "unlawful attempt to deny pro-life Americans their First Amendment rights".

"In a brazen affront to the First Amendment, Massachusetts government officials had sought to use the threat of arrest and criminal conviction to silence those offering women life-affirming alternatives to abortion,” said Charmaine Yoest, president of Americans United for Life.

The Massachusetts branch of Planned Parenthood, America's most prominent pro-choice group, said the decision showed "a shocking disregard for the safety of patients and staff".

Planned Parenthood vowed to immediately return to Massachusetts's Democrat-controlled state legislature to pass a new law to protect women but also conform to the Supreme Court's ruling.

Abortion remains a bitterly divisive issue in the United States and protesters regularly gather at clinics to try to dissuade women from going through with the procedure.

Eight people have been murdered at abortion clinics since 1977, according to the National Abortion Federation, a pro-choice group. The statistics also record 42 bombings and more than 33,000 arrests.

The push for buffer zones in Massachusetts stems from a 1994 shooting when John Salvi III, a 22-year-old pro-life activist, killed two receptionists at abortion clinics in Brookline, a suburb of Boston.

The nine justices that sit on America's highest court usually divide into liberal and conservative camps on controversial decisions, with centrist Justice Anthony Kennedy casting the deciding vote.

However, in today's ruling all nine justices voted to strike down the law, saying Massachusetts had failed to strike a balance between free speech and public order.

The ruling leaves state authorities room to craft a narrower law that could prevent demonstrators from directly approaching women but still leave them free to express their opposition to abortion.

Massachusetts was the only state to pass such a strong buffer-zone law but the ruling is also likely to strike down similar laws passed in liberal cities like San Francisco and Portland, Maine.

Although all nine justices voted to strike down the law, the court's more conservative members filed their own concurring opinion, saying the ruling did not go far enough.

"Today’s opinion carries forward this Court’s practice of giving abortion-rights advocates a pass when it comes to suppressing the free-speech rights of their opponents," wrote Justice Antonin Scalia, one of the court's most conservative figures.